Mode-locked semiconductor lasers are attractive sources of picosecond pulses for numerous applications, e.g., high bit rate optical communications, ultrafast photonic switching, optical radar and time-resolved electro-optical sampling. Although semiconductor lasers have been previously mode-locked by passive, synchronous, hybrid and colliding pulse techniques, active mode-locking is the most commonly used mode-locking technique. Active mode-locking is accomplished by modulating the gain of the semiconductor laser in an external cavity at a frequency equal to or an exact integer multiple of the frequency spacing between the longitudinal modes, and is most frequently used to generate picosecond pulses. Active mode-locking is most effectively achieved when the transit time of the external cavity matches exactly with the period of the modulation current. For several applications, e.g., in digital communications links, optical clocking of high speed digital computers and sampling measurements, the noise characteristics of the pulse train in terms of amplitude or pulse energy fluctuations and phase or pulse time jitter are of prime importance and together with the pulse duration of the laser, effectively define the maximum available bit rate and sampling bandwidth respectively. Unfortunately, in the case of active mode-locking even a slight detuning between the RF modulation frequency and the cavity transit time results in a phase shift between the modulation and the mode-locked pulse train, which leads to instabilities in the mode-locking process, degrading both the pulsewidth and the stability of the output. Such a degradation can result in the repetition rate of the pulses fluctuating with time. Hence, both a well-stabilized laser cavity and a highly stable (and usually expensive) RF signal generator are typically required for conventional active mode-locking of semiconductor lasers.
Examples of active mode-locking with reasonably high stability but using relatively expensive equipment such as ultrastable RF synthesizers are described, e.g., by Burns et al. in "Noise Characterization of a Mode-Locked InGaAsP Semiconductor Diode Laser" (IEEE J. Quantum Electron., Vol. 26, pp. 1860-1863, 1990) and by Derickson et al. in "Residual and Absolute Timing Jitter in Actively Mode-Locked Semiconductor Lasers" (Electron. Lett., Vol. pp. 2026-2028, 1990).
Other examples of mode-locking systems are described in "Self-mode-locking of a semiconductor laser using positive feedback", by Derickson et al. (Appl. Phys. Lett., Vol. 56, pp. 7-9, 1990), wherein a mode-locking technique called self-mode-locking was described using a high-speed optical to electrical converter in a positive feedback configuration to convert the output optical pulses back into electrical drive signals in a regenerative process, such a self-mode-locking including both coordinated optical and electrical feedback to obtain short pulses, and in "Mode Locking of a Semiconductor Laser by Self-Synchronising Optoelectronic Feedback of the Longitudinal Mode Beats" by Nietzke et al. (Electron. Lett., Vol. 26, pp. 1016-1018 (1990) wherein self-synchronised mode locking was achieved by optoelectronic feedback of amplified RF beats of the longitudinal modes of an external cavity semiconductor laser, which RF was superimposed onto the DC injection current to obtain short pulses. Similar optoelectronic feedback schemes have been employed for gas and solid state lasers. While these systems can be considered regenerative mode-locking, there is no indication that either of such systems has the stable repetition rate that would be necessary for numerous applications.
Accordingly, it is an object of the present invention to provide a high stability mode-locked short pulse laser source without the use of an external highly stable RF signal generator tuned to an integral or subintegral multiple of the resonance frequency of the laser cavity.
It is another object of the present invention to provide a high stability short pulse source for electromagnetic radiation without the use of a highly stable signal generator tuned to an integral or sub-integral multiple of the resonance frequency of the cavity containing an amplifier of that electromagnetic radiation.
Still another object of this invention is to provide a method of generating short pulses from a highly stable mode-locked laser source.
Yet another object of this invention is to provide a method of generating short pulses from a highly stable source of electromagnetic radiation.